One of the methods currently used for operations of refueling and fuel transferring comprises a telescoping tube, which in turn, comprises a fixed part and a mobile part, having the mobile part the capacity to be extended telescopically with respect to the fixed part, carrying out in this mariner the transfer of fuel from the fuel tank of the tanker aircraft to the fuel tank of the receiver aircraft.
For the specific case of the telescoping boom, the operator needs to fly the boom up to introduce the tip of the telescopic boom, usually called nozzle, into the receiver receptacle. Once the nozzle has made contact some latches ensure that the nozzle is engaged into the receptacle. At this specific instant it is essential that automatic systems detect the latching condition to commute from the previous state “no contact/free air” to the “contact/coupled” state.
Indeed, the flight control laws of refueling booms with load alleviation system capability embodied on the aerial refueling boom system itself are different if the boom is in “free air” condition or “coupled” condition. In the first case the flight control laws permit to maintain the boom stable within a predetermine envelope, i.e., the area in which contact with the boom is safe, depending on the flight conditions and action of the air refueling operator on the flight control stick. In the second case, during a contact between tanker and receiver aircraft and “coupled” condition, the flight control laws will aim to reduce and alleviate as much as possible the loads produced by the contact boom—receiver. For that, the air refueling boom system flight control laws will be limited on “accompanying” the movement of nozzle into the receptacle, as this one is moving back-down-up-downwards.
The need to identify the contact status is therefore essential for aerial refueling through the aerial refueling boom system, as the flight control laws could be applied unintentionally but in a proper way and in accordance with its intended function once the nozzle is in contact, but when the “coupled” status is not transmitted properly. Additionally, the contact status is also normally used to manage the fuel pumps of the refueling system.
The actual state of the art uses a unique method to detect the nozzle connection status that is based on an induced signal that transmits an electrical pulse corresponding to the change of status from “not connected” to “connected/coupled,” and vice versa. Moreover, once the nozzle is engaged in the receptacle, the nozzle also provides the capability for signal and voice communication between the tanker and receiver aircraft via the mated nozzle and receptacle induction coils and the aircraft's communication systems.
These systems nowadays used in air to air refueling operations are based on the transmission of signals between two induction coils, one located in the boom nozzle and the other one located in the receiver's receptacle. Indeed, when the nozzle is engaged in the receptacle, both coils are aligned, allowing the signal to be transmitted form tanker to receiver aircraft (both signal and voice communications) and vice versa. The receiver aircraft sends the signal of “contact” to tanker aircraft, which change the mode from “free flight/no contact” to “coupled/contact.” The transmitted signal comprises a voltage, usually comprised in the range of 7.5 to 30 V, which is then amplified by both tanker and receiver signal amplifiers. This system also has the capability to transmit signals when both coils are not fully aligned, when the nozzle assembly is inserted and the nozzle and receptacle coils are rotated (generally up to 15 degrees, either clockwise or counterclockwise, relative to each other) and separated by a maximum air gap.
From the tanker perspective, the latches of the nozzle are by default in a mechanically locked configuration which provides a rigid attachment point for the engagement of receptacle toggle latches. An independent disconnect system equipping some nozzles permits a command for the disconnection from the tanker side, by releasing the force applied to the latches and permitting the nozzle to be extracted from the receptacle. At the same time, an induced signal is transmitted to the receiver aircraft via an induction coil in order to declare the disconnect status.
From the receiver perspective, the receiver receptacle is also equipped with hydraulically commanded latches that apply a force on the latches of the nozzle in order to maintain it in contact position. When the latch valve control is energized, the latch cylinder will move the latch shaft and, therefore, the latches will close, fixing the nozzle. If the tensile strength of the latches is above a determined threshold, the relief valve will crack, relieving the hydraulic pressure in the latch cylinder, retracting, therefore, the latches.
A contact switch is activated when the nozzle is inserted. The nozzle depresses a sliding valve assembly in the receptacle, which, in turn, actuates the contact switch. As the switch is actuated, electrical power is applied to the toggle latch control valve which provides hydraulic power to the toggle latch actuator. The latch switch activation occurs when the toggles close to latch the nozzle into the receptacle. The actuation of the latch switch provides an electrical pulse to the receptacle signal amplifier which advances it to the contact made position. The connect light is then illuminated in the cockpit. Voice communication is allowed through the boom nozzle and the receptacle's induction coil.
On the completion of the refueling, a disconnect signal sent from either the aircraft causes the latch actuator to extend and release the boom nozzle. Disconnect signals are sent to both aircraft to illuminate respective indications. When in disconnect, the receptacle is configured so it cannot again latch the nozzle. In case other contacts were attempted, the universal aerial refueling receptacle slipway installation (UARRSI) can again reset to ready status by a dedicated pushbutton in the cockpit, which actuates directly over the amplifier.
As described above, during a contact or disconnect phase, the main features involved in this process are the induction coil of the nozzle, the induction coil of the receiver, the latches of the nozzle and the latches of the receiver. Unfortunately, several failure scenarios can be identified as will be explained below.
Regarding mechanical failures:
1. In case the nozzle is inserted into the receptacle, but the latch (either tanker or receiver) is failed, the coils will be aligned and the signal “contact/coupled” will be transmitted, while the nozzle will remain uncoupled. This scenario is known as “float out,” and has only operational impact on the operation, but no safety impact.
2. In case the nozzle is inserted into the receptacle and is unable to be removed due to receiver latch failure if the tanker is equipped with an independent disconnect system, disconnect can be controlled by the tanker aircraft. If not equipped, there are two backup means for accomplishing disconnection: a relief valve internal to the latch actuator of the receptacle, designed to open at a crack pressure equivalent to a system specified tension load, or a controlling shear section built into the torque shafts. The failure point is controlled to a location, which allows the torque shaft torsion spring to retract the latches for subsequent stiff boom refueling if required.
Regarding electrical failures:
1. If a failure is declared in the amplifier, the override function, which is enabled by a dedicated pushbutton in the cockpit of the receiver aircraft, will de-energize the amplifier and will establish an alternative path to arm the latch valve, so when the nozzle enters, the valve can be energized. In the disconnection, when in normal mode, the latch valve line is de-energized. The disconnection signal can come either from the receiver or the tanker side. In override, the amplifier is disabled, so the disconnection signal can only arise from the receiver side following other way, which will energize a relay that keeps the latch valve de-energized continuously. In nozzle with IDS (independent disconnect system) function, the disconnection can be commanded by tanker.
2. In case the receiver contact pulse fails, the nozzle will be engaged into the receptacle, whereas the tanker aircraft refueling laws will remain as “free flight, no contact.” This represents the worst scenario, as the loads on the nozzle will increase significantly. This failure case has not only an operational, but also safety impact on the refueling operation.
As a consequence, it has been explained that the state of the art systems have a lack of reliability while detecting the signal contact/no contact, given the main issue that these existing methods usually have is that the reliability of the pulse reception is very low, as it is based in an induced signal. Moreover, and in case the pulse is correctly transmitted (no failure of coil), it has been seen that the status could be erroneous, as a “contact” status with two coils aligned does not ensure the nozzle is properly inserted and latched into the receptacle.
It is also known from the published U.S. Patent Application US2012/0305710 that a significant approach is made in terms of detecting the contact/no contact condition by adding additional sensors. In particular, this system adds:
A device for detection of the insertion of the nozzle into the receptacle.
A device for detection of the state of the latches of the receptacle.
A device for detection of the state of the latches of the nozzle.
Additionally, that application describes a device that permits the measurement of the displacement of a piston with a ramp to close or open a cutoff valve of the nozzle. The device comprises a proximity sensor attached to the body of the nozzle, and would constitute an additional means of detection of the insertion of nozzle inside the receptacle.
The main advantage of all these devices is that they are all embodied in the tanker side, concretely in the nozzle, and do not require any modification of the receptacle of the receiver. However, their implementation presents several challenges due to the limited physical space inside the nozzle and due to the explosive atmosphere surrounding the nozzle. Moreover, a single device for detection of the state of the latches of the nozzle does not seem to be a fully reliable solution. Indeed, the document indicates that when the independent disconnect system is commanded, the pistons of the nozzle collapse, in order to activate a microswitch. However, experience has revealed that the pistons do not collapse when the independent disconnect system (IDS) is commanded, but are simply released and loosen. As consequence, the signals “IDS activated/nozzle latches free” would not automatically match if they were compared between IDS signal and microswitch signal.
This highlights some lacks in the disclosure presented in US2012/0305710 A1:
The solutions presented, although present evident improvements for contact/no contact condition, might not be easy to implement in the nozzle.
Their reliability would not be ensured, in particular in the case of the microswitch detecting the state of the latches of the nozzle.